1. Field of the Invention
This invention relates to structures employing semiconductor powders and thin catalyst films which can photosensitize chemical reduction-oxidation (redox) reactions.
2. Prior Art
FIG. 1 shows a schematic diagram of a prior art, conventional photoelectrochemical cell 10 in which a semiconductor 11 of macroscopic dimensions is connected by an external circuit 12 to a counter electrode 13. Both semiconductor 11 and counter electrode 13 are immersed in an electrolyte solution 14, advantageously aqueous, which contains chemical species (redox species identified as D- and A+) which are subject to either oxidation or reduction reactions that occur with the transfer of electronic charge at the surfaces of semiconductor 11 and counter electrode 13.
Light of energy greater than the energy of the semiconductor electron bandgap is made to illuminate the semiconductor. As a result, pairs of free electrons 15 and holes 16 are generated near the illuminated surface. Under appropriate conditions for the energy level position of the redox levels in the electrolyte relative to the conduction and valence band energy positions in the semiconductor, the electrons and holes can transfer to the redox species thereby effecting the redox reactions. The various processes and conditions involved are discussed by H. Gerischer in Physical Chemistry--An Advanced Treastise, H. Eyring, D. Henderson, W. Jost. Eds. (Academic Press, New York, 1970), pp. 463-542.
FIG. 1 illustrates a typical situation for an n-type semiconductor where surface electric fields caused by the equilibration between electrolyte redox levels and the semiconductor Fermi level draws photoinduced holes to the semiconductor surface to oxidize redox species D- to D while electrons pass through the external circuit to reduce A+ to A at the counter electrode. At steady state, the electronic current I.sub.e in the external circuit is matched by an equal and opposite ionic current I.sub.i in the electrolyte. In summary, the semiconductor acts as a photosensitizer for carrying out the reaction D-+A+.fwdarw.D+A. For example, TiO.sub.2 sensitized photodecomposition of formic acid and other carboxylic acids in aqueous environments is well known.
Such photoelectrochemical cells have a number of applications. Using the example of FIG. 1, if the reduced species is at a higher energy than the oxidized species, there is a net storage of incident radiant energy as chemical energy, as in a photoelectrosynthetic cell. The photoelectrochemical splitting of H.sub.2 O is an example. If the reduced species is at a lower energy than the oxidized one, no energy has been stored. Rather the photoelectrochemical system has catalyzed a thermodynamically downhill reaction. An example is the photocatalytic decomposition of acetic acid to methane and carbon dioxide.
In a regenerative cell, the species oxidized at the semiconductor is also reduced at the counter electrode so that there is no net change in the energy stored in the electrolyte. However, the current in the external circuit can be used to drive an electrical load, as in a photovoltaic cell. These and other device possibilities have been described by Bard (A. J. Bard, Science 207, 139 (1980)).
One advantage of the photoelectrochemical cell over the solid state photovoltaic cell is that it is produced quickly by simply immersing the semiconductor in the electrolyte. Secondly, light is absorbed in the region of the surface electric field of the semiconductor which causes the electron and hole to separate before they recombine. In solid state cells, the high electric field separation region is frequently at a greater distance from the surface of the semiconductor. This necessitates higher quality and higher cost material so as to allow the carriers to diffuse to this region before they recombine.
A microscopic version of a prior art photoelectrochemical cell is shown schematically in FIG. 2. Here microscopic semiconductor powder grains 20 are dispersed in an electrolyte solution 21 again containing redox species D- and A+. Typically, these powder grains are submicron in dimension. The external circuit and counter electrode of FIG. 1 are replaced by a piece of catalytic material 22, typically a metal such as platinum, attached to a region of the semiconductor. The attachment can be accomplished by photochemical means. When the powder dispersion is illuminated, processes occur similar to those described for the macroscopic system of FIG. 1. Holes are drawn to the semiconductor surface where they oxidize D- while electrons move to the catalyst region where they reduce A+. The electronic flow in the grain amounts to an electronic current I.sub.e while the current loop is completed in the electrolyte by ionic current I.sub.i. Thus, the grains act as microscopic short-circuited photoelectrochemical cells. The powder has the advantage of providing much more reactive surface area. A disadvantage is not having an external circuit essential for photovoltaic and other electrical device applications. Writings by Gratzel et al (J. Kiwi, K. Kalyanasundaram, and M. Gratzel, Structure and Bonding 49, Springer-Verlag, Berlin, p. 37, (1982)) and Bard (A. J. Bard, J. of Photochem. 10, 59 (1979)) discuss many of the details of the microscopic system and the numerous modifications that can be attempted to make the system more effective under different circumstances.